Field of the Invention
The invention relates generally to circuit interrupter apparatus and, more particularly, to apparatus providing undervoltage control of an associated circuit interrupter.
Description of the Prior Art
Circuit breakers are widely used in industrial, commercial, and residential applications to provide protection for electrical apparatus and distribution equipment. Upon overcurrent conditions through a connected circuit, the circuit breaker will automatically open to interrupt current flow through the circuit. Certain applications such as in the mining industry require an undervoltage release capability, allowing the circuit breaker contacts to be automatically opened whenever a control voltage falls below a predetermined minimum. This capability is often provided by an electromagnet which restrains a plunger positioned in proximity to a trip mechanism. So long as the electromagnet is energized, the breaker will be maintained in an untripped condition. Upon de-energization of the electromagnet, a bias spring operates the plunger against the trip mechanism to automatically open the circuit breaker contacts.
An undervoltage control for molded case circuit breakers is required to hold in, that is, to refrain from tripping the breaker, when the input control voltage rises above a certain fraction (between 35% and 75%) of its nominal value. The device must be able to withstand at least 110% of nominal voltage continuously, according to Underwriters Laboratories Standard 489. Because of the wide range of input control voltage, these devices are subject to problems such as marginal hold-in capability at low voltage and excessive heat dissipation by the electromagnet coil at high voltage. To overcome these problems, series transistor regulators have been used to supply constant current to the coils. This type of regulator requires power to be dissipated in a transistor which must be mounted on a relatively large and expensive heat sink. Also the extra power dissipated inside the circuit breaker housing is undesirable. Switching regulators, which dissipate negligible amounts of power, have not been used because of the complexity of the required control circuitry.
It would therefore be desirable to provide undervoltage control apparatus for a circuit breaker which combines the relatively low cost and simplicity of the series transistor regulator with the low power dissipation of a switching regulator.